When these noun+ly adjectives like timely, leisurely, homely, slovenly, motherly, and others get used as adverbs, it's hard to hear that as anything but symptomatic of erosion of language skills.
It's just a linguistic reflex, albeit an instilled one, to hear any adjective+ly in an adverbial sense and any noun+ly in an adjectival sense.
She motherly put the children to bed? Not hardly. ;)
When an IRS publication says that filing timely can reduce the risk of penalties, we make unflattering assumptions about the person who wrote that. More generally, such usage generally manages to suggest underdeveloped discernment.
Yet some dictionaries bless "leisurely" and possibly others of its ilk for adverbial use. Who knows, maybe such usage can be traced far back in English history. It nonetheless grates on reflex and on insight into how these words relate to -lich -lig -lik cousins in Germanic and Scandinavian languages. It's really hard not to spontaneously feel that if "couples strolled leisurely in the park" is not symptomatic of modern erosion of meaning, then it must be evidence of historical erosion of meaning.
So while, out of respect for authority, I won't write traffic citations for misuse of "leisurely," I'll nonetheless stick to a structurally consistent usage myself.
Now if we could just figure out where to put "comely"...